Handle missing Author or Maintainer fields in library.properties nicely

[per1234]

There is no reason to break all compiles just because one of these fields is missing from any installed library. Instead handle it in the same way as a missing category and just default to Unknown.

[ffissore]

One of the reason behind library.properties is to provide library users enough information to contact library authors and get feedback. I understand this may be a little boring, but it ensures support requests are sent to the proper guys